The process of cartographic reproduction, i.e. map reproduction, requires exacting tolerances for the faithful reproduction of visual character information from a master, or source chart. Faithful reproduction of character resolution is necessary to ensure the accuracy in depicting the placement and distances of the various topography features, to permit reliable calculation of distances, bearings etc. for accurate navigation. The Hydrographic/Topographic Center of the United States Government's Defense Mapping Agency (DMA) has the responsibility of providing to both Government and public agencies maps of both domestic and foreign countries. The foreign source material is derived from foreign counterparts of the DMA and varies greatly in the types of symbology including thickness of line defined boundaries and topographic legends and the methods of providing color deposition and color contrast.
The source cartographic data may be provided in various color combinations and may include an opaque base on which the visual characters are drawn, or in some cases a transparent base. The exacting resolution requirements, which are designed to ensure the detection of the leading and trailing edges of the various boundary lines (line thicknesses) force the reproducer to work with the smallest of resolution elements on the master chart. The ability to discriminate between colors in a multicolor master lithograph requires exacting color discrimination tolerances. The source chart colors distinguish, or denote changes, highlights, or hazards of the hydrographic/topographic surfaces. This may include various shades of a same base color in distinguishing surface topography or depth of water. In addition the use of different, but similar colors to distinguish national boundaries or navigational hazards requires the ability to discriminate between similar color tones. Finally, the processes used in depositing the color vary, and include even tone color, screen color, process color, or a mix of one or more on a single chart. The process color method creates a color from the combination of dots of a few primary colors, differing in dot size and screen angle; the resulting color visually sensed is the sum of the component dots. To obtain, instrumentally, the same color recognition the chart area from which the input signal is produced must be large enough to yield an equally distinctive result as that of the visual response. This of course is contrary to the requirement for the smallest resolution element to be scanned to satisfy the resolution accuracy requirements. Similarly with screen color the density of color is established by the dot size, i.e. the area covered by ink per unit of area on the chart. The recognition of process colors and screen colors impacts the optical system by the requirement for the relatively large area signal measurement, at the same time requiring the resolution-scale input measurement for even tone (solid) color graphic data which may also exist with screen method or process method color presentations.
The prior art cartographic reproduction methods include both a strictly manual process and also a manual digitizing technique, which are used to translate the visual information from the lithograph master into one or more lithographic plates; each representing one level of information from the chart, such as the black and white element information followed by one or more lithographic plates dedicated to reporting within the various boundaries. The prior art manual techniques are considered necessary due to the high accuracy requirements. The individual lithographic plates produced are then transformed through photographic etching techniques into press plates, each representative of the particular level of chart reproduction information. The strictly manual and manual digitizing techniques are both labor intensive and time consuming; in some instances the reproduction of a full size chart requiring almost a full man year.
In the manual digitizing methods, the apparatus used includes an objective lens with a fixed field stop; the objective lens being used with a source of illumination which is concentrated on the surface of the chart. The objective lens receives a portion of the illumination reflected back from the chart, and forms in the image plane of the lens the resolution area image which is limited in aperture size by the field stop setting. To satisfy the high resolution accuracy of the graphic characters, or symbology, a small field stop value is necessary; typical values used in cartographic digitizing are of a size necessary to provide a 0.025 to 0.1 millimeter aperture diameter (including objective lens magnification) in the image plane. The imaged information is then presented to spectral separation and filtering apparatus which detects the color of the imaged chart area, after which associated electronic equipment identifies the color of the image area from the spectral component measurements. The use of the flux field stop settings restricts the usefulness of the optical recording apparatus, and its universal applicability, limiting the equipment to specific types of cartographic materials.
Since the field stop is selected to produce resolution size images for detection of edge transitions in the graphic symbology at the required resolution accuracy, for the large area color screen printing the resulting digitized data of the smaller resolution size aperture is forced to record each color dot and the spaces between dots. This greatly increases the volume of output data stored, in addition to increasing processing time and computer capacity.
Another limitation of the fixed field stop (fixed resolution aperture) cartographic raster scanners of the prior art is the variation in thicknesses between different source charts. The thickness of the source chart results in a change in the optical path length between the source chart surface and the incident surface of the objective lens. As a result the sensed image may not appear, or at least not appear with the required resolute accuracy, in the image plane of the object lens. In addition the magnification of the sensed image is also changed with changes in working distance. All of this contributes to errors in the detected positional accuracy of edge transitions on the source chart, in the registration of the position of geographic coordinates of the source chart, and in the size of the chart area included in the field stop aperture setting.